Oh boy! I mean, berk!
I've developed quite an interest in the chaotic planes between the alignments. The ones that seem to never really get much attention. Beastlands, Ysgard, Pandemonium, Carceri, and Gehenna. (Law is boring.)
I think 3 out of 4 times an outer plane appears in D&D, it's the Abyss. If it's not the Abyss, it's either Baator, Limbo, or Celestia. I've almost never seen anything being done with any of the other planes.
Similarly, I quite like the yugoloths. For exactly the same kind of reasons.
Occasionally you get Carceri because of Torment and the Demodands.
But for me it's Aborea, Tartarus (I prefer this name to Carceri), Hades, Ysgard, Archoron (keep mixing this name up with Archons, the Celestials), Mount Celestia, Elysium, and Limbo.
Pandomium, Gehenna, Biopia, Arcadia just seem so seem so pointless, like two have some biblical inspiration, but nothing that doesn't seem better covered by Hell and the Abyss. Nothing that makes them pop. The other two seem to not even have Mythological inspiration, they are kind of boring. They need some cool stuff, they feel like filler.
Also the the middle kinds of planes that are partly neutral, but also partly not neutral, like what does that even mean on practical terms, I mean Ysgard is Chaotic Good like Aborea, but also Chaotic Neutral like Limbo. It seems like this needs better explanation.
Best faction of Sigil is Society of Sensation, but despite liking Planescape as my second favourite classic setting, alot of the faction feel poorly conceived.
For creatures, I'll stick to stuff that hasn't been reveal or that I think won't end up in 2024 MM. An update to Nymphs, Lillend, Ausuras, Brachina, Lilitu, Tartarus Titans.